Superoutbursts of Z Cha and their Interpretation

Disk eclipses of Z Cha during its superoutbursts are analyzed. The resulting flux/temperature distributions in the outer parts of the disk show peculiarities which are interpreted as being due to absorption effects in the overflowing parts of the stream. The temperatures of the inner parts of the disk are used to determine the accretion rates as a function of time since superoutburst maximum. They are practically identical with mass transfer rates determined from the hot spot. Results are summarized in the form of the following, purely observational scenario: Superoutbursts are due and begin with a major enhancement in the mass transfer rate. During the "flat-top" part of the superoutburst the mass transfer rate decreases slowly, causing the observed luminosity to decline. The superoutburst ends when the mass transfer rate decreases below its critical value, resulting in a transition to the quiescent state of the dwarf nova cycle.